During adolescence the brain undergoes significant maturation, and thus is thought to be especially vulnerable to the neurotoxic effects of alcohol. Past research has shown that heavy alcohol use during adolescence is associated with abnormal hippocampal structure, function, and impaired cognition. In contrast to alcohol use, aerobic exercise has been linked with improved cognition, and has been shown to be neuroprotective against the detrimental effects of alcohol on the hippocampus in animals. These findings suggest exercise may be beneficial to the hippocampus in youth with alcohol use disorders (AUD). However, since little research has investigated the impact of aerobic fitness on cognition and brain function in healthy children and teens, it is necessary to first determine if the benefits of exercise established in animals translate to human adolescents. Thus, to better understand how exercise may influence the brain and behavior during adolescence, this cross- sectional study will examine the impact of exercise on hippocampal structure and function, as well as cognition. Specifically, the proposed study will use actigraphy and aerobic capacity testing, as well as functional and structural magnetic resonance imaging, neuropsychological testing, and spatial memory assessment to examine hippocampal structure and function in high-active and low-active male youth. Understanding how exercise impacts the brain may be the first step in determining how exercise-induced plasticity reported in the animal literature may translate to humans and may ultimately permit the development of strategies aimed at reducing cognitive deficits that are seen in adolescents with AUD.